1. Field of the Invention
The invention relates to a permeable conductive shield providing protection from electromagnetic interference and electrostatic discharge, and in particular, to such a permeable conductive shield having a simple laminated structure suitable for application to the foil side or the component side of a printed circuit board.
2. Background Information
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. Such radiation often develops as a field or as transients within the radiofrequency band of the electromagnetic spectrum, i.e., between about 10 kHz and 10 GHz, and is termed “electromagnetic interference” or “EMI” as it is known to interfere with the operation of other proximate electronic devices. To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed to either confine the EMI energy within a source device, or to shield an affected device from other devices that are sources, or a combination of both. Such shielding is provided as a barrier that is interposed between the source and the affected device, and typically is configured as an electrically conductive housing that encloses the device, often as a “can” that covers a discrete component or group of components of the device. The housing can be formed of a metal or alternatively of a plastic material which is filled so as to be electrically conductive, as described for example, in U.S. Patent Publication No. 2004/017502, or which may be provided with a conductive coating generally applied across the surface of the housing, as described, for example, in U.S. Pat. No. 6,483,719.
Electrostatic discharge, or “ESD,” refers to the damage that can occur to electronic components when subjected to an electrical static charge, particularly when being handled during installation or test. Some types of computer circuitry, such as semiconductor memory devices, are particularly sensitive to ESD. A common procedure for avoiding ESD is to ground the individual handling the circuitry. A more foolproof method is to enclose the circuitry in a conductive shield that is electrically connected to the circuit ground, as described, for example, in U.S. Pat. No. 6,884,937. Thus, the same conductive shield can serve to protect from both EMI and ESD.
Another problem experienced with the commonly used types of EMI shields is the build up of heat in the components being shielded. One solution that has been utilized to solve this problem is to add heat-dissipating fins to the outside of the shielding enclosure, as described in U.S. Pat. No. 6,884,937. But this solution adds to the fabrication cost of the enclosure, increases its size and weight, and is certainly not an effective solution unless some sort of metallic, heat conducting, material is used.
Thus, what is needed is a simple and easily manufactured conductive shield suitable for providing EMI and ESD protection to the foil or component side of a electronic circuit board while allowing the dissipation of heat generated in the circuits to which the shielding is applied.